56 Oil the Mineralogtcal Geography 



history by saying, tliat it is frequently exposed on the sur- 

 face of the earth, but frequently also it is covered with ar*- 

 gillaceous marles, with freestone without shells, and finally 

 with alluvial soil. Such is the nature of the soil of ihfr 

 forests of Foniainebleau, 



Art. VII. Formation of Freestone without Shells, 



The freestone without shells, in whatever place it is 

 found, is always cither the last orihe penultimate formation. 

 It constantly covers the rest, and is never covered, except 

 by the formatii^n of alluvial soil. Its beds are frequently 

 very thick, and mixed with beds of sand of the same na- 

 ture. The sand which ?upp(^rts the upper beds, has been 

 sometimes washed out by the water ; the beds arc then 

 broken, and have rolled over the flanks of the hillocks which 

 they formed : of this kind arc the freestones of the forest 

 of Fontainebleau, of Palaiseau, Sec. 



Not only do this freestone and sand contain no fossils, 

 but they are frequently very pure, and furnish sands much 

 esteemed in the arts, and which are gathered at Etarrjpes, 

 Fontainebleau, Aumor , occ. 



They are sometimes, however, either altered by a mixture 

 of argil, or coloured by oxides of iron, or impregnated 

 with carbonated lime which has penetrated them by infil- 

 tration when they are covered by the ca^lcarcous soil of fresh 

 V<iter: this is still the case with the freestone of several 

 parts of the forest of Fontainebleau » 



Art. VIII. Formation of the Fresh-water Soil, 



This formation constantly covers all the foregoing. The 

 rock which has resulted from it resembles, \n pomt of struc- 

 ture and other external properties, siliceous limestone, i, e. 

 it is sometimes compact, sometimes white and soft, but al- 

 most always penetrated with siliceous infiltration. The 

 name silex, soujetimcs opaque and yellowish, sometimes 

 brown and translucid like pyromatic silex, in some places 

 completely fills the place of the limestone: finally, this 

 formation gives, like the sixth, burrstones, the origin of 

 which has one and the same cause. 



What exclusively therefore characterizes this formation is, 

 on the one hand the }iresencc of shells, which are evidently 

 of fresh-water origin, and similar in every resj^ect to those 

 which we find in our marshes. These shells are lymnere 

 and three kinds of planorbes. We also find in this forma- 

 tion 



